Voltage Divider Calculator
A voltage divider uses two resistors in series to produce a lower output voltage from a higher input. Enter any three of the four circuit values and this calculator solves for the fourth. Choose what to solve from the dropdown, pick resistor units (ohms, kilohms, megaohms), and optionally add a load resistor to see the loaded output voltage. Current through the divider and power dissipated across each resistor update instantly.
Formula
Worked example
Converting 5 V to 3.3 V: Vout = 5 × R2 / (R1 + R2) = 3.3 V. Choosing R2 = 2 kΩ, rearrange: R1 = R2 × (5 - 3.3) / 3.3 = 2000 × 1.7 / 3.3 = 1030 Ω. Divider current = 5 / 3030 ≈ 1.65 mA. Power in R1 = 1.65² × 1030 ≈ 2.8 mW; power in R2 = 1.65² × 2000 ≈ 5.4 mW.
How a voltage divider works
A voltage divider is one of the simplest and most widely used circuits in electronics. Two resistors are connected in series between a supply voltage (Vin) and ground. The junction between them taps off a fraction of Vin as the output voltage (Vout). Because the same current flows through both resistors, Ohm's law gives Vout = Vin x R2 / (R1 + R2). The ratio of R2 to the total resistance sets the fraction: if R2 = R1, Vout is exactly half of Vin; if R2 is larger than R1, Vout is more than half; if R2 is smaller, Vout is less than half. Real-world design also requires you to know how much current the divider draws from the supply (quiescent current = Vin / (R1 + R2)) and how much heat each resistor must shed, because an undersized resistor can overheat and drift out of tolerance.
Solving for any unknown - the four modes
This calculator lets you solve for any one of the four circuit variables. Solve for Vout is the most common mode: you know the supply voltage and your two resistors and want to verify the output. Solve for Vin is useful when you are working backward from a required output and a pair of resistors already on hand. Solve for R1 or R2 is the design mode: you set a target Vout from a known Vin, fix one resistor to a convenient standard value, and the calculator gives you the exact value of the other. Because the result may not be a standard part, you then pick the nearest E24 or E96 series value and re-enter it to confirm the real-world Vout is still inside your tolerance.
Loading effects and why they matter
The formula Vout = Vin x R2 / (R1 + R2) assumes no current is drawn at the output. In practice, any load connected across R2 - a microcontroller input, an ADC, a sensor, or another circuit - forms a parallel combination with R2, reducing the effective lower leg resistance and pulling Vout below the no-load value. The loaded Vout is Vin x (R2 || RL) / (R1 + R2 || RL) where R2 || RL = R2 x RL / (R2 + RL). As a rule of thumb, keep RL at least ten times larger than R2 to hold the loading error below 10%. For better than 1% error, make RL at least 100 times R2. This calculator shows both the no-load Vout and the loaded Vout, plus the percentage drop, so you can choose resistor values that keep the error within your specification.
Choosing resistor values in practice
There is a trade-off between power consumption and load sensitivity. Low-value resistors draw more quiescent current, which wastes energy but makes the divider stiff - less affected by load. High-value resistors use almost no standby power but are sensitive to any downstream load. For battery-powered designs, values in the 100 kΩ to 1 MΩ range minimise idle drain. For logic-level conversion between active ICs, 1 kΩ to 10 kΩ is typical. Always check the power rating: a 1/4 W resistor can handle up to 250 mW; confirm that P = I2 x R stays comfortably below that figure. For precision work, use 1% metal-film resistors rather than 5% carbon-film types, and note that resistance drifts with temperature (typically 50 to 200 ppm per degree Celsius for standard parts).
Common voltage divider applications
| Application | Typical Vin | Typical Vout | Recommended R range |
|---|---|---|---|
| 5V to 3.3V (logic level) | 5 V | 3.3 V | 1 kΩ - 100 kΩ |
| 12V to 5V (ADC reference) | 12 V | 5 V | 1 kΩ - 47 kΩ |
| Battery monitoring (9V) | 9 V | 4.5 V | 10 kΩ - 1 MΩ |
| Sensor signal scaling | Variable | Microcontroller max | 10 kΩ - 1 MΩ |
| Potentiometer trimmer | Variable | Adjustable | 1 kΩ - 100 kΩ |
| Biasing transistor base | 5-12 V | 0.6-2 V | 10 kΩ - 470 kΩ |
Typical use cases and recommended design constraints.
Frequently asked questions
What is a voltage divider circuit?
A voltage divider is a simple circuit with two resistors in series across a supply voltage. The output is taken at the junction between them. Because the same current flows through both resistors, the output voltage is a fixed fraction of the input: Vout = Vin x R2 / (R1 + R2). Voltage dividers are used for logic-level conversion, biasing transistors, creating ADC reference voltages, and building sensor interfaces.
Can a voltage divider supply current to a load?
A voltage divider can supply a small load current, but it is not a voltage regulator. Any current drawn by the load reduces the effective resistance of the lower leg, which lowers Vout. For a stiff output that barely changes with load, keep the load resistance at least 10 times larger than R2, and ideally 100 times larger. If you need a well-regulated voltage under varying load, use a linear voltage regulator or a buck converter instead.
How do I convert 5V to 3.3V with a voltage divider?
Using Vout = Vin x R2 / (R1 + R2), set Vin = 5 V and Vout = 3.3 V. With R2 = 2 kΩ, R1 = 2000 x (5 - 3.3) / 3.3 = 1030 Ω. The nearest E24 value is 1 kΩ, giving Vout = 5 x 2 / 3 = 3.33 V - close enough for most digital inputs. Enter these values in the calculator above to verify current and power dissipation before committing to a PCB layout.
What resistor values should I use?
The ratio of R1 to R2 determines Vout; the absolute magnitude determines quiescent current and power. For microcontroller circuits, 1 kΩ to 100 kΩ balances low power with low susceptibility to loading. For battery-powered or low-power designs, 100 kΩ to 1 MΩ reduces idle current at the cost of higher loading sensitivity. Always verify that the power in each resistor (I2 x R) is well below the resistor's rated wattage, typically 1/8 W or 1/4 W for through-hole parts.
Why does Vout change when I connect a load?
The load connects in parallel with R2, reducing the effective lower resistance. A smaller lower leg means a lower voltage fraction. The effect is significant when the load resistance is comparable to or smaller than R2. To reduce loading error, use smaller resistors (lower impedance divider) or buffer the output with an op-amp voltage follower, which presents a very high impedance to the divider and a very low impedance to the load.
What is the difference between a voltage divider and a potentiometer?
A potentiometer is a variable voltage divider: it is a single resistor with a sliding contact (wiper) that divides the total resistance into two parts. The wiper position sets the ratio, giving an adjustable output voltage. A fixed voltage divider uses two discrete resistors and gives a fixed ratio unless the resistors are changed.
How much power does the voltage divider waste?
Total quiescent power = Vin2 / (R1 + R2), or equivalently I2 x (R1 + R2). For a 5 V divider with 1 kΩ + 2 kΩ, power = 25 / 3000 = 8.3 mW. For a 100 kΩ + 200 kΩ divider at the same ratio, power = 25 / 300000 = 83 µW - 100 times less. Use higher resistors where quiescent power matters.